This invention relates to a method and apparatus for transmit power control. In particular, this invention relates to a method and apparatus for setting the energy at which a transmit power control command is transmitted.
Good transmit power control methods are important to communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. For example, transmit power control is necessary to obtain high system capacity in third generation communication systems that use wideband code division multiple access (W-CDMA). The goal of transmit power control is to use as low a transmission power as possible, thereby minimizing interference, while providing the desired quality. This is important for the uplink, i.e., for transmissions from a remote terminal to the network, e.g., a base station, as well as for the downlink, e.g., for transmissions from the network to the remote terminal.
Power control is commonly provided by a closed-loop method. For example, for uplink power control, a base station determines whether the uplink power should be increased or decreased based, e.g., on measured uplink signal strength, and then transmits an appropriate power control command to the remote station at regular intervals, e.g., every 1.25 milliseconds. Based on the power control command, the remote station increases or decreases its uplink transmit power by a predetermined amount. The power control command is typically transmitted as a bit, the value of which determines whether the transmit power of the remote station should increase or decrease, e.g., by a specified step. For example, a “zero” power control bit transmitted by the base station may cause the remote station to increase its transmit power level by 1 dB, and a “one” power control bit transmitted by the base station may cause the remote station to decrease its transmit power level by 1 dB.
The decision about which power control command to send to the remote terminal from the base station may be made by measuring the quality, e.g., the signal to interference ratio (SIR), of the received uplink signal and comparing the measured SIR with a reference SIR that represents an acceptable quality. If the measured SIR is lower than the reference SIR, the base station sends a command to increase the uplink transmit power to the remote terminal. If the measured SIR is above the reference SIR, the base station sends a command to decrease the uplink transmit power to the remote terminal. The remote terminal receives the command and changes its uplink transmit power, accordingly. A detailed example of uplink power control is provided in the TIA/EIA/IS-95-A standard.
Similarly, for downlink transmit power control, the remote terminal measures the quality, e.g., the SIR, of the received downlink signal, and the remote terminal transmits a report of the measured SIR or a power control command to the network, e.g., to the base station that the remote terminal is communicating with. Based on such a report or command, the power level (and thus the SIR) of the downlink signal is appropriately controlled by the network, e.g., the base station or another network entity, such as a radio network controller (RNC).
In a W-CDMA system, such as the Third Generation Partnership Program (3GPP) system, time is split into slots. In each slot, both data and control information are transmitted. Control information may include, for example, known pilot symbols and transmit power control (TPC) commands. The TPC commands that are sent on the downlink are used for power control of transmission on the uplink, and the TPC commands that are sent on the uplink are used for power control of transmission on the downlink. The downlink TPC commands inform the remote terminal how to change its transmit power, and the uplink TPC commands inform the base station how to change its transmit power.
An example of a 3GPP slot structure is shown in FIG. 1. Each slot includes the following fields: Transport Format Combination Indicator (TFCI), Data 1, TPC, Data 2, and Pilot. The TFCI field includes information regarding how the data is coded. The Data fields include the data being transmitted. The TPC field contains one or more TPC commands used for controlling the transmit power. The Pilot field contains information used for synchronization. Each of these fields may contain one or more symbols, the number of symbols depending on the implementation. The symbols may be repetition coded, e.g., the symbols may be repeated to ensure correct transmission. More details of an exemplary slot structure are given in “Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, Technical Specification No. 3G TS 25.211, ver. 3.2.0, 3GPP (March 2000), herein incorporated by reference.
Conventionally, the transmit energy of the TPC command itself is not varied independently of the transmit power control. That is, there is currently no provision for individually setting the energy at which the TPC command is transmitted. The TPC command is transmitted at the same energy level that is set for transmission of other types of information. Thus, the TPC command transmit energy is not changed based on which command is sent or how important it is that the TPC command be received without transmission error.
If the measured SIR is very close to the reference SIR, it is not critical that the remote terminal receive commands from the network to increase or decrease the transmission power, because such commands would likely not affect the quality of the received signal. In such a case, the transmit energy of the TPC command could be decreased, without an adverse effect. This would decrease interference, thus increasing the system capacity. On the other hand, if the SIR is far below the reference SIR, it could be very important that a command instructing the remote terminal to increase the transmit power reach the remote terminal without transmission error. Otherwise, the call could be dropped. If the measured SIR is far above the reference SIR, it could also be important that a command to decrease the transmit power reach the remote terminal correctly to keep the interference caused by the remote terminal as low as possible.
Thus, there is a need for setting the energy at which a TPC command is transmitted based on how important it is that the TPC command is correctly received.